A U.S.-backed Kurdish-Arab alliance from the north and Russian-backed Syrian troops from the southwest are set to catch Islamic State of Iraq and Levant (ISIL) in a pincer movement as they advance on the terror group’s stronghold at Raqqa.

U.S. military officials said the Syrian Democratic Forces (SDF) alliance has captured more than 100 square kilometers (40 square miles) of territory from ISIL this week as it moves toward Raqqa.

SDF Manbij operation.

Meanwhile, Syrian government troops, backed by Russian air support, advanced within dozens of kilometers of the Euphrates Valley town of Tabqa, site of the country’s biggest dam, the Syrian Observatory for Human Rights said.

The dam is some 50 kilometers (30 miles) upstream from Raqqa. It was the first time that Assad regime forces had entered Raqqa province since they were ousted by ISIL in August 2014.

Observatory director Rami Abdel Rahman told AFP that “it seems there has been an undeclared coordination between Washington and Moscow.”

Russian Foreign Minister Sergei Lavrov discussed Syria with U.S. Secretary of State John Kerry in a telephone call on June 4, the foreign ministry in Moscow said, without giving details.

Further up the Euphrates, SDF forces launched an assault on the strategic Manbij area on the Turkish border, regarded as a key entry point for ISIL’s foreign jihadists.

Washington has deployed more than 200 special forces troops in support of the Kurdish-led SDF, which the U.S. regards as the most effective fighting force on the ground against ISIL in Syria.

The SDF’s offensive in Manbij pocket is aimed at seizing the last stretch of border still under ISIL control and denying the jihadists any opportunity to smuggle in recruits and funds.

ISIL responded with a rare offensive of its own against two towns held by anti-Assad rebels further west in a bid to enlarge the territory it holds on the border.

Washington has dropped ammunition to the rebels defending the town of Marea in a bid to stop ISIL from overrunning it, a U.S. official confirmed.

The supply lines from neighboring Turkey have made the northern border region one of the most contested battlegrounds of Syria’s five-year civil war.